Cautious optimism perhaps best describes the mood of NASA engineers and astronauts, awaiting the launch of the first space shuttle mission in more than two years. The shuttle Discovery is scheduled to lift off from Kennedy Space Center in the southeastern U.S. state of Florida Wednesday. It's the first shuttle mission since February 2003, when the Columbia shuttle broke apart while reentering orbit, killing all seven crew members.

As NASA prepares for Discovery's liftoff, there are reminders of the Columbia tragedy in every aspect of the upcoming mission. In January 2003, a large chunk of insulating foam smashed a hole in Columbia’s left wing during liftoff. That damage was blamed for the shuttle's disintegration.

Discovery will carry seven astronauts to the international space station, along with essential supplies and replacement parts.

The mission has been adjusted, to include measures aimed at preventing another tragedy.

While in flight, crewmembers will test the new shuttle safety systems, monitor the condition of the shuttle, and practice maneuvers needed to carry out necessary repairs.

Professor John Logsdon is the Director of the George Washington University Space Policy Institute. He served on the Columbia accident review board and believes NASA has improved shuttle safety.

"I think it is going to be intensely emotional and everybody is going to hold their breath until -- you can't do it for the multiple days of the mission -- but until the shuttle is safely back on the ground at Kennedy Space Center at the end of the mission, people are going to be very nervous about the success of this because the stakes are so high," said the professor.

If something does go wrong with Discovery, the astronauts will dock the shuttle at the International Space Station. Discovery's crew would then wait for another shuttle to come get them. But the space station barely has enough food, water, and oxygen for its own crew of two.

Discovery astronaut Wendy Lawrence admits, "We know the number of days and the estimate is the space station could support us for 43 days."

The rescue vehicle would be the space shuttle Atlantis. But it would take at least 35 days for Atlantis to be ready for lift off from Kennedy Space Center.

The commander of the Atlantis rescue mission is Steve Lindsey. "This is the first time that we ever planned, had a planned rescue in our hip pocket, if you will. Hopefully, what it shows is that we are really trying to make this next one as safe as we possibly can."